The present embodiments relate to a preprocessing circuit for receive signals of a local coil for a magnetic resonance (MR) facility. Proposals have been made to undertake part of processing of receive signals in or at least in the region of a local coil for magnetic resonance circuits.
Local coils are increasingly used in magnetic resonance circuits to record magnetic resonance images with a high signal-to-noise ratio. The excited nuclei in the object to be recorded induce a voltage in the coil conductor of the local coil. The voltage generally is amplified using a low-noise amplifier (LNA) and then forwarded by cable connection at the magnetic resonance frequency (transmit frequency) to an input of a receiver of the receive system. To improve the signal-to-noise ratio further even with high-resolution magnetic resonance images, high-field systems are used. The basic field strengths of a high-field system are around three Tesla or higher.
Local coils are generally connected to a patient couch by way of suitable plug-in points. Because the patient couch may be moved around, cable connections meters long are frequently used to forward the received signals to the receive system. Local coils may include a number of individual coil elements (loops), each supplying a receive signal, so that wired transmission links are generally required for each of these so-called receive channels.
An arrangement for transmitting magnetic resonance signals received with the aid of local coils is known from DE 10 2008 023 467 A1. It is proposed there in one embodiment that two receive signals are transmitted, in other words multiplexed (frequency multiplexing), by way of a single connecting line. It is also proposed there that low intermediate frequencies may be used to transmit the receive signals. The intermediate frequencies are selected so that the intermediate freqeuncies are located symmetrically around the sampling frequency of the downstream analog/digital converter.
To convert the receive signals to the intermediate frequencies, a preprocessing circuit is provided. The preprocessing circuit includes at least a first mixer and at least a second mixer. The mixers operate as frequency converters and use two different auxiliary frequency signals (local oscillator signals). The auxiliary frequency signals are supplied to the preprocessing circuit by a coaxial cable. The coaxial cable also is used to transmit the receive signals (with an intermediate frequency) to a receiver of the receive system of the magnetic resonance facility. Therefore, one or more auxiliary frequency signals may be transmitted to the local coil on one coaxial cable and one or more magnetic resonance signals (each transformed to an intermediate frequency) may be transmitted away from the local coil.
The requirements specified for the transmission link from the point of view of auxiliary frequency signal processing are less stringent than the requirements resulting from the point of view of intermediate frequency signal processing. At the feed point, line attenuation may largely be compensated for by increasing the feed power. However, as the frequency of the auxiliary frequency signal increases, more and more problems may result due to motion- and/or time- and temperature-dependent phase responses and reflections (level uncertainties) caused by discontinuities (e.g., plug-type connectors).
In the example from DE 10 2008 023 467 A1, the required auxiliary frequency is calculated as the magnetic resonance frequency (transmit frequency) +/− the intermediate frequency. From auxiliary frequencies of around 150 MHz upwards, the described effects make it necessary to adjust the transmission link so that the advantages described by DE 10 2008 023 467 A1—thin and low-cost cable design, use of low-cost, for example non-coaxial plug-type connectors, narrowband design of switches to select certain coil elements—are increasingly negated.
If, for example, the receive system with its intermediate frequencies disposed with mirror symmetry in relation to a 10 MHz sampling rate as described in DE 10 2008 023 467 A1 is to be used for operation with a seven Tesla magnetic resonance system, an auxiliary frequency signal pair of 285 MHz and 305 MHz results. At such auxiliary frequencies, the advantages gained by using the intermediate frequencies may be lost again.
Other types of preprocessing may also be provided by a preprocessing circuit at the local coil. The other types of preprocessing may also use the transmission of auxiliary frequency signals.